Microsoft Windows 7 problem 'could affect millions'

Microsoft has been investigating reports that some computers running Windows 7 crash as soon as the user logs on

Users have been complaining on internet forums about the "black screen of death", which causes the screen of their Windows 7 machine to turn black and the computer to crash when a user logs on.

Microsoft confirmed that it was investigating the possibility that a security update, released on Thursday, could be the root of the problem but later said that it was not the cause.

"Microsoft has investigated reports that its November security updates made changes to permissions in the registry that that are resulting in system issues for some customers," a statement read.

"The company has found those reports to be inaccurate and our comprehensive investigation has shown that none of the recently released updates are related to the behavior described in the reports."

It added: "Our support organization is also not seeing this as an issue."

The "black screen of death" also appears to affect other Windows operating systems, including Windows 7's predecessor, Windows Vista, as well as Windows XP. When users log on, they see a completely black screen instead of the usual start menu, desktop icons and system tray.

According to the software firm Prevx, which has issued a patch to resolve the problem, millions of computer users could be affected by the "debilitating" glitch.

"Users have resorted to reloading Windows as a last-ditch effort to fix the problem," said the company in a blog post.

"The cause appears to be a change in the Windows operating system lockdown of registry keys," said Dave Kennerley, a support engineer with Prevx. "This change has the effect of invalidating several key registry entries if they are updated without consideration of the new ACL (access control list) rules being applied."

Microsoft advised those affected by the problem to contact its customer service line. A spokesman said the problems didn't match any existing known issues. Microsoft is yet to release a patch to resolve the problem.